VARA HF , TRI MODE and the SDR #SDR #VARA


Graham
 

VARA HF  , TRI MODE and the SDR

The   use of SDR and the  VARA HF modem is  limited to  , SDR  one side of the  connection  only 
ie Conventional <> SDR  , or  SDR <> Conventional 

Where as VARA FM and  VARA Sat , support SDR<>SDR 

As V modem deploys dynamic timing , adjusting for symmetrical & asymmetrical   
link  delays , this at  first sight, looked a little  'odd 
considering, the  FM and Sat versions are 100% compatible 

It transpires that  VARA HF latency cannot be  extended , due to the  
requirement to  be compatible with  TRI Mode  band  scanning limitation  of 4 seconds 
in which, the delay from only, one  SDR  may  be  accommodated ..this may not 
be  fully recognised  and  could  present  un-expected  issues  ..

Implications :- 

VHF 'FM'  1.2/9.6  Unaffected   
V-SAT   unaffected 

HF gateways  deploying  SDR may  fail to  connect  with  SDR  clients
conventional tx/rx may be  required  to  provided  conventional & sdr connections 

VARA-HF-Chat  SDR<>SDR may not  function  *

*NB-  VARA -SAT  will  function with  Chat  SDR<>SDR  
over HF link - 

Unless the  SDR  processing times  are   reduced  and/or  the  4 second  Tri-mode scan  
issue is addressed , the situation  remains precarious , with legacy issues influencing
HF gateway configuration .. extending the  V-HF latency [accommodation] is at odds  with 
the  Tri-Mode  scan  cycle  ... catch 22 

73-Graham
g0nbd


James Clark
 

Are Winlink aware of this issue? And do they plan to address it?


Graham
 

Ok  James's 

Good  question, 

The Trimode  Dwell  time  of   4 seconds ,  is the  'only' limiting  factor , preventing the  deployment  of   SDR   in a   gateway 
which is perhaps a legacy issue .. 

Unlike 'fixed'/'static'  timed  systems / protocols , Processing  delays  and  switching times  are  not critical,  the  Vara  modem  deploys  'Dynamic' timing , additional delays [symmetrical & asymmetrical]
are automatically  compensated  for . eg  repeater  delays  for  VHF/FM and the  long  path delay  , for the AO-100   Sat link version  [ which also  can be used for  HF etc ]  , leaving the  HF version, while  compatible with the  4 second  dwell requirement ,unable to provided  otherwise,  sdr<>sdr connectivity 

Established: 

The  limiting  factor  is  solely the  4 seconds  delay , [as to  why  '4' seconds  is required
is not  clear ?] .  

Increasing the  Dwell  time  by  500mS  , to  4.5  seconds , would  allow  deployment of the   Vara HF
modem , inside the  Trimode  system and  provide  both  SDR<>SDR and  Hybrid connectivity 
both  inside and  out side the  winlink  system.

73-Graham
g0nbd




Graham
 
Edited

SDR latency is a little longer  than conventional ,  But , as is ,  its not possible  for , say,
a two  way connection , using , Flex SDR<> Flex SDR  and the  Chat module 
probably a case of  unexpected consequence .. 

Clarification   from  Jose , 

Jose EA5HVK

3 May 2021, 18:48:23 (yesterday) 
to VARA HF Modem
Yes, with Dwell Trimode 4.5 seconds it would be possible to do a simple change to VARA HF to connect two SDRs: SDR<->SDR.   Although the latency in this case would be about 1 second due to both VAC. 
 
But Winlink demanded 4 seconds maximum Dwell, so only is possible  SDR<->Conventional  or  Conventional <->SDR.   
 
In VARA FM or VARA SAT is possible SDR<->SDR because there is no DWELL time requirements.  
 
Jose


73-Graham
g0nbd